president obama now says that the full weight of the u.s. government is behind the search for who is responsible. >> authorities this morning are pouring over this dramatic video at the finish line for clues, bomb technicians are studying bun marks on bricks and concrete and in hospitals doctors are collecting pieces of shrapnel that were blasted into bodies. >> they will look at the components and tell them if this is someone that put this together over the internet or is this a bombmaker? >> officials believe these were two small and portable devices, possibly hidden in a backpack, one working theory they were remotely detonated by a cell phone the. >> it has the indications of being al qaeda or al qaeda-affiliated attack. >> there are no suspects. officials are tracking multiple "persons of interest" including a man of middle eastern descent being treated at a local hospital. president obama urged against a rush to judgment not using the word "terrorism" but vowed justice for those responsible. >> make no mistake we will get to the bottom